With locked rate lenders not honoring full original rate buydown

[Cody A.]

Hi. I'm under contract on a new construction home. We locked rates for the loan a little over a month ago when rates were around 6%. Where with the home builders closing incentives and a little extra cash we were going to be able to buy the rate down to 4.75%. Now that we are two weeks out the lender is saying that even though we are still in the rate lock timeline that the coupons for many of the rate buydown options from when we locked are gone and they can't honor those rates anymore and will only let us buy down to 5.25%. 

Is this something they can actually change with us having locked the rates or should they have to honor the full rate buydown table we were given at the time of locking rates?